The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) will produce forefront science by quantifying the response of the magnetosphere to the time variable solar wind. It will acquire, for the first time, a variety of three-dimensional images of magnetospheric boundaries and plasma distributions extending from the magnetopause to the inner plasmasphere. The images will be produced on time scales needed to answer important questions about solar wind - magnetosphere interactions.

In its report, "Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015," the Task Group on Solar and Space Physics of the Space Science Board identified four key questions for magnetospheric physics:

  1. How does the solar wind couple to the magnetosphere?
  2. What is the origin and fate of magnetospheric plasmas?
  3. How is energy stored and released in the magnetic tail?
  4. How does the magnetosphere couple with the atmosphere and ionosphere?
The report noted that the only practical post-ISTP approach to testing of global magnetospheric models is one that utilizes techniques to provide global images of the magnetosphere. In the subsequent Space Physics Strategy Implementation Study [1991], the Inner Magnetosphere Imager (IMI) mission was given highest priority for magnetospheric physics with the realization that the techniques of neutral atom imaging and ultraviolet imaging of He+ at 30.4 nm are well enough developed to provide global images of regions such as the ring current and plasmasphere.

During the study phase, a strawman payload and model spacecraft design were identified to accomplish the IMI science objectives. Developing needs for lower-cost spacecraft led to a final report on a descoped mission, Magnetosphere Imager (MI), which still embodied the critical techniques of neutral atom imaging (1 keV to 200 keV) and 30.4 nm imaging, along with imaging of the global aurora and the geocorona. This mission would indeed yield the first global views of the magnetosphere, specifically of the build-up and decay of the ring current and the erosion and refilling of the plasmasphere, along with the contextual measurements and link to ISTP that would be provided by auroral imaging.

As defined, MI would fall short of answering two key questions identified in the Space Science in the Twenty-First Century report to which it is potentially very relevant: 1. How does the solar wind couple to the magnetosphere? and 2. What is the origin and fate of magnetospheric plasmas? Question 1 requires quantitative measurements of the structure of the magnetopause, while question 2 requires images of both the ionospheric source and the solar wind source of magnetospheric plasma. MI would make no measurements of the magnetopause, nor would its lower energy limit for magnetospheric ion imaging (1 keV) be low enough to trace the ionospheric plasma back to its source. On the other hand, our proposed IMAGE mission, while accomplishing the complete MI objectives, will also utilize two techniques recently developed by the IMAGE team to address effectively both of these questions.

The first technique is radio plasma imaging (RPI), which senses plasma densities between 0.1 and 105 cm-3 and images plasma boundaries globally throughout the magnetosphere on a one-minute time scale. Thus, for example, the global topology of the magnetopause and the structure of its interior boundary layer will be imaged continuously as they respond to changes in the solar wind and interplanetary magnetic field, addressing a crucial aspect of question 1 above.

The second new technique employed by IMAGE is the extension of neutral atom imaging down to an energy of 10 eV by using a surface conversion technique for converting low-energy neutral atoms into ions for energy and mass analysis. This low energy coverage is essential for addressing the origin of magnetospheric plasmas. The ionosphere is the source of a large portion of magnetospheric plasmas, and the injection of ionospheric ions into the magnetosphere can only be traced by imaging down to these energies.

With its unique instrument complement, the IMAGE mission will address the high-priority magnetospheric physics objectives in ways not accessible to the MI mission. It will obtain global images from the magnetopause to the aurora, sensing changes in solar-wind conditions as reflected by magnetopause topology and tracing both the solar-wind source and the ionospheric source of magnetospheric plasmas. The IMAGE instrumentation has been chosen because it has optimum performance in every parameter range, has been developed by experienced scientific teams with extensive successful flight history, is flight-proven to the maximum extent possible, and has been prototyped and verified on the ground wherever new techniques are required.

The IMAGE spacecraft has been designed by GSFC using off-the-shelf components from the FAST spacecraft, with a GSFC/Resource Analysis Office (RAO) official cost estimate well below the MIDEX requirement. The IMAGE mission represents a very low risk MIDEX with very high scientific payoff. It complies with all MIDEX requirements for spacecraft resources (mass, power, and data rate), mission schedule, mission costs and contingencies. The IMAGE Science Team will claim no proprietary rights and is committed to providing open access to all IMAGE data and an extensive program of education and public outreach based directly upon programs already implemented by IMAGE team members.


In-situ measurements over the past 35 years have yielded a wealth of statistical information about the magnetosphere and its constituent plasma regimes. They have also provided many examples of dynamical changes of magnetospheric plasma parameters at specific times and places in response to changes in the solar-wind input and to internal disturbances related to substorms. Statistical global averages and individual events are, however, not sufficient to understand the dynamics and interconnection of this highly dynamic system. Fundamental questions concerning plasma entry into the magnetosphere, global plasma circulation and energization, and the global response of the magnetospheric system to internal and external forcing remain unanswered. These processes occur on time scales of minutes to hours, yet currently available statistical averages are on time scales of months to years. Statistical studies, by their nature, never represent the magnetosphere at any instance in time. For example, the statistical magnetopause topology derived by Sibeck et al., [1991] was obtained from an accumulation of approximately 20 years of boundary crossings by a variety of spacecraft. However, the magnetopause is not well represented by these average profiles. To further our understanding of the physical processes that affect the magnetopause requires the nearly instantaneous measurement of its topology.

To address such basic questions, IMAGE will provide global imaging of three general magnetospheric regions: a) the magnetopause, boundary layers, cusp, and auroral zone; b) the plasmasphere; and c) the inner plasma sheet, ring current, and trapped radiation. In addition, it will provide images of near-Earth interplanetary space. The data acquired in each of these regions will be used to determine the global structure of the magnetosphere, characterize the connectivity between magnetospheric regions, identify dynamic responses in these regions, and place results in a global context with previous in-situ measurements.

The overall objective of IMAGE is best expressed by the question: How does the magnetosphere respond globally to the changing conditions in the solar wind? In fact, with all its implications, this question is a statement of the fundamental problem facing magnetospheric physics. Unlike other disciplines such as astrophysics, solar physics, and to a partial extent ionospheric physics, magnetospheric physics has not had the benefit of a global perspective of the constituent regions under study. IMAGE will provide this perspective for the first time. Specific questions around which the IMAGE mission has been designed are:

1) What are the dominant mechanisms for injecting plasma into the magnetosphere on substorm and magnetic storm time scales?

2) What is the directly driven response of the magnetosphere to solar wind changes? and

3) How and where are magnetospheric plasmas energized, transported, and subsequently lost during storms and substorms?

The aim of the IMAGE mission is to address these objectives in unique ways using existing imaging techniques: neutral atom imaging (NAI) over an energy range from 10 eV to 200 keV, far ultraviolet imaging (FUV) at 121 - 180 nm, extreme ultraviolet imaging (EUV) at 30.4 nm, and radio plasma imaging (RPI) over the density range from 0.1 to 105 cm-3 throughout the magnetosphere. These techniques are referred to by their initials in the following science discussion, which is used to specify the performance requirements of the IMAGE instruments. Figure 1.2.1 illustrates the type of magnetospheric image data that these four techniques will acquire.

Fig. 1.2.1 Simulated IMAGE data from RPI, FUV, NAI, and EUV.

1.2.1 Mechanisms for Injecting Plasma into the Magnetosphere

a. Solar wind plasma entry. In-situ measurements have revealed the general structure of the magnetopause, established that it is almost continuously in motion, and detected the existence of a boundary layer, consisting of a mixture of magnetosheath and magnetospheric plasma with densities intermediate between the two regions. Magnetosheath plasma has been shown to be accelerated as it crosses the magnetopause and to flow relatively unimpeded through the polar cusps and down into the ionosphere [e.g., Smith and Rodgers, 1991; Fuselier et al., 1991].

These observations have led to the general agreement that magnetic reconnection is important along the magnetopause. However, the relative global importance of reconnection, which produces abrupt gradients within a boundary layer of variable thickness, and diffusive processes, which lead to generally smooth density gradients within a boundary layer of uniform thickness, is very uncertain. An intermediate case with a boundary layer on both closed and open field lines has been discussed by Lotko and Sonnerup [1994], while Song et al. [1993] show that for northward IMF the low-latitude boundary layer (LLBL) shows a stair-step plasma density profile with no evidence for plasma flow between the steps. The ability of RPI to make global determinations of the plasma density profiles through the boundary layer and magnetopause is crucially important for gaining an ultimate understanding of magnetopause plasma entry processes.

Using HEOS-2 data, Sckopke et al. [1976] found on average a thicker plasma mantle for northward IMF; Mitchell et al. [1987] determined statistically that the LLBL on the flanks is thicker for southward IMF. However, it has never been possible to compare the thicknesses of the mantle and the LLBL simultaneously to determine if the entry of solar-wind plasma shifts between high and low latitudes as the IMF changes. With RPI this important simultaneous measurement will be possible, as will the determination of whether or not the boundary layer becomes thicker around the flanks of the magnetosphere, as claimed by Mitchell et al. [1987] and contested by Phan and Paschmann [1995].

Recent studies indicate that the cusp at various times can be successfully modeled either by plasma entry during quasi-steady reconnection [Onsager et al., 1993] or by pulsed reconnection [Lockwood and Smith, 1992]. At present the relative importance of these two extremes is unclear because temporal and spatial variations cannot be resolved by single in-situ measurements along a spacecraft trajectory nor can they be resolved by statistical studies. With the ability to image the density enhancement contained within the cusp with RPI, the cusp ion population with NAI, and the electron and proton auroras associated with the cusp with FUV, both a steady cusp and a pulsating cusp can be resolved and characterized. In the pulsed case a series of discrete density enhancements will be observed in the cusp while for the quasi-steady case the enhancement will be more continuous.

b. Ionospheric ion upwelling. Another important source of plasma is the ionosphere. The source locations of the ionospheric outflows and their relation to the local and global ionospheric plasma and magnetic structures and dynamics are not yet fully understood. Moore et al. [1985] have suggested that an intense localized cusp provides the ionospheric ions for the magnetosphere, while Shelley et al. [1985] describe the outflow region as a diffuse and extensive region comprising the entire auroral oval. This controversy results from the dependence up to now on statistical surveys, which use months of in-situ data and fail to resolve spatial and temporal features on the needed time scale of minutes.

Figure 1.2.2 shows a simulated image of the charge-exchanged O+ outflow from the cusp as will be measured by NAI. Consecutive images (with several minute resolution) will determine the ion outflow flux and composition down to energies of 10 eV as functions of magnetospheric activity (as defined by the FUV images of the auroral activity and the RPI measurements of the magnetopause).

Fig. 1.2.2 (Top) simulated neutral atom image of ionospheric ion outflow. The peak flux is approximately 105 cm-2sr-1s-1. (Bottom) Ionospheric ion outflow model results. Contours of O+ fluxes flowing parallel to B are shown in units of 106 cm-2s-1 down (+) and up (-) the field line, respectively. [Horwitz, 1986].

1.2.2. Directly Driven Response of the Magnetosphere to Solar Wind Changes

a. Magnetopause erosion and Kelvin-Helmholtz instabilities. The most immediate effects of the solar wind on the magnetosphere arise from its interaction with the magnetopause. Despite extensive evidence for the inward displacement, or erosion, of the dayside magnetopause, along with a predicted flaring of the tail magnetopause during the substorm growth phase [Coroniti and Kennel, 1972], it has not been possible to measure the global position of the magnetopause on the several-minute time scale needed to confirm these effects. RPI can measure the magnetopause global position and the cusp position while FUV monitors substorm activity, hence testing the theoretical and statistical predictions concerning magnetopause erosion and its association with substorm activity.

Quasi-regular bright spot structures are observed in the auroral oval afternoon sector. These auroral phenomena are part of a larger set believed to be connected with large-scale surface waves on the LLBL [e. g., Lui et al., 1989], which are predicted to be caused by a Kelvin-Helmholtz (K-H) instability [e. g., Melander and Parks, 1981]. Establishing this connection between the LLBL and the aurora and clearly identifying sources for these auroral structures require the simultaneous measurement of auroral morphology and magnetopause boundary layer structure and motion. RPI measurements of the boundary layer structure [Fung et al., 1995] and irregularity scale size can be compared with periodic auroral structures observable simultaneously by the FUV imager to determine various properties of the predicted K-H instabilities, e. g., their spatial extent along the magnetopause.

Other phenomena, such as flux transfer events (FTEs) and pressure pulse perturbations on the magnetopause, may have similar (but currently undiscovered) signatures in the ionosphere. For example, Lockwood and Smith [1992] have identified the pulsed cusp with FTEs, and the combination of NAI imaging of the time-varying cusp ions with RPI imaging of boundary layer structure will establish any connection between these two regions.

b. Enhanced current systems. Increased tangential stresses are imposed upon the magnetopause when the southward IMF component and/or the solar wind velocity increase. These stresses lead immediately to field-aligned currents connecting the magnetopause to the ionosphere (the dayside region-1 currents). The currents are continued across the polar cap ionosphere and thence by internal field-aligned currents (the region-2 currents) throughout the magnetosphere. The internal currents are associated with plasma pressure gradients, particularly in the plasma sheet and ring current. While the low-altitude field-aligned currents have been computed globally from magnetometer measurements, only very isolated measurements have been made of the magnetospheric currents. Imaging of magnetospheric ion populations will allow the computation of global current densities both perpendicular and parallel to the magnetic field, i. e., the ring current and the region-2 currents, as discussed by Roelof [1989] for isotropic pitch angle distributions.

Neutral atom images will contain information on the pitch angle distribution of the ring current ions. This information can be used to assess the current distribution in the magnetosphere, according to

An example of this result is shown in Figure 1.2.3. For steady conditions, the parallel current can be derived from this perpendicular current by requiring that J = 0, and the full 3D current flow field can be inferred.

Figure 1.2.3. The azimuthal current density in the equatorial plane and in the noon-midnight meridian Derived from a 3D ring current simulation based on the model of Fok et al. [1995].

c. Plasmasphere erosion and refilling. The plasmapause is traditionally considered to be the boundary between closed and open convection trajectories. Accordingly, simple models indicate that when the open/closed convection boundary moves inward, filled plasmasphere flux tubes become entrained in sunward convection flow to the magnetopause and form long tails [Rasmussen et al., 1993].

Observations suggest a more complicated picture in which outlying high- density regions may be detached from the plasmasphere [Chappell, 1974]. Internal low-density regions may be a consequence of erosion by particle precipitation, suggesting that as much plasma can be lost from the plasmasphere by precipitation as can be lost by convection. Other observed dynamical features that are difficult to reconcile with the traditional picture include nightside plasmapause steepening with increasing Kp [Chappell et al., 1970] and rapid radial motion of density boundaries across broad MLT sectors.

Magnetospheric imaging should be able to solve the problem of the time-dependent structure of the plasmasphere as follows. EUV will measure the global distribution of He+ in the inner magnetosphere in sequences of two-dimensional, line-of-sight images. RPI will identify internal density structures such as biteouts, closely wrapped tails, or field-aligned density structures, which would otherwise be obscured in integrated EUV images. Ring current images from the NAI will identify ring current-plasmasphere interactions, and the entire set of images will be placed in context with magnetospheric activity through FUV observations of auroral morphology.

1.2.3 Energization and Loss of Magnetospheric Plasmas

a. Ring current injection. McIlwain [1974], using ATS-5 data, suggested that large transient electric fields inject fresh particles into a region outside a sharp boundary during substorms. Later Sauvaud and Winckler [1980] showed the impulsive, dispersionless plasma injections observed by ATS-1 and ATS-6 to be associated with the magnetic reconfiguration of the tail toward a more dipolar configuration. Moore et al. [1981] then used ATS-6 and SCATHA data to measure plasma injection front velocities in the range of 10-100 km/s, which they attributed to the induced electric field of the earthward propagating compression waves observed by Russell and McPherron [1973]. Moore et al. [1981] were able to use dual-satellite measurements to determine that the electron injection signature is produced mainly by a boundary motion rather than by local acceleration of plasma. However, some evidence was found for plasma heating, which accumulates during a series of injection front passages. Recently Jacquey et al. [1993] associated earthward-moving injection fronts with a disruption of the cross-tail current at 6-9 RE, which propagates tailward with a velocity of 150-250 km/s and also propagates longitudinally during the substorm expansion phase.

Figure 1.2.4 Simulation of a ring current injection in the nightside magnetosphere, in 1.7 keV ion flux (left), neutral atom flux (center), and ENA image counts (right), for a 120 sec. exposure by NAI from the IMAGE orbit.

The ionosphere contributes significantly to the energetic ion population of the magnetosphere. For example, Daglis et al. [1990] demonstrated a large enhancement of O+ ions in the near-Earth nightside magnetosphere during the substorm growth phase. However, in order to determine the respective roles of ionospheric ion injection, in-situ ion acceleration, and earthward ion transport during substorm injections, it will be necessary to obtain composition-resolved images of the ion populations in the near-Earth plasma sheet and ring current regions over an energy range extending from very low values characteristic of the ionosphere up to energies of tens of keV. Figure 1.2.4 shows the results of a simulated ring current injection for a magnetic storm. The three panels show the ring current ions, the neutral atom population resulting from charge exchange, and the image that would be obtained with the NAI instrumentation on IMAGE.

b. Ring current dissipation. Storm energy, initially resident in the ring current, is lost to energetic neutral atoms and to the aurora. The charge exchange process between ring current ions and the geocorona is believed to be the dominant loss mechanism. Pitch-angle diffusion of ring current ions, leading to precipitation, is due to collisions with the low-energy plasma of the plasmasphere and to wave-particle interactions. Through correlated study of auroral images from FUV, plasmasphere images from EUV and RPI, and hot plasma images from NAI, an assessment can be made of the importance of plasma waves in the loss of the ring current.

1.2.4 Interstellar Neutrals and Coronal Mass Ejections (CMEs). Although primarily magnetospheric imaging techniques, NAI and RPI will acquire important data on phenomena exterior to the magnetosphere.

a) The isotopic composition of interstellar H and He. Most attempts to determine the interstellar neutral composition use the charge-exchanged component to deduce the interstellar density. NAI will make the first direct measurements of the isotopic composition of interstellar H and He neutrals at Earth. These neutrals are distinguished from ambient magnetospheric neutrals by their arrival direction (from the solar apex direction) and low energy (~10-200 eV). Long-term measurements will allow the first direct detection of the deuterium abundance in the local interstellar gas, which is important for understanding the early history and geochemical evolution of the solar system and interstellar medium.

b) Use of CME-related neutral H fluxes and type II radio bursts for early warning of geomagnetic storms? CMEs cause geomagnetic storms as they pass the Earth. As they propagate away from the Sun, CMEs produce high energy neutrals [Hsieh et al., 1992a] and Type II radio bursts, some of which travel ahead of the CMEs. NAI will detect the CME-produced neutral H flux at keV energies. Similarly, RPI observations of Type II radio bursts will occur up to 40 hours before CME arrival. Thus, IMAGE will offer the previously unavailable early warning of severe geomagnetic storms that seriously upset spacecraft, power grids, and communications.


Our approach to the IMAGE investigation has been to evaluate the science objectives with extensive modeling in order to establish a set of performance parameters for the various imaging techniques (NAI, FUV, EUV, and RPI). In some cases (FUV and NAI), more than one sensor technology is required, and the different sensors are combined within a single instrument. An optimum polar orbit with 500 km perigee and 7 Earth radii apogee altitudes, which is initially at a latitude of 45 degrees north in the dusk meridian, has been chosen. Finally, we have adopted an integrated mission philosophy (Fig. 1.3.1), which begins with an existing spacecraft (FAST), within which the entire IMAGE payload is accommodated as a single instrument in terms of the electrical interface. The orbital strategy is to have only two data acquisition modes (high altitude and low altitude) so that mode commanding is done automatically based on orbital position. During mission operations the data will be acquired by the SMEX ground station and immediately formatted for distribution through Internet to the entire scientific community, with no proprietary data rights. Building upon education and public outreach programs already underway by IMAGE investigators (Connections, P. Reiff; INSPIRE, W. Taylor), we will provide interesting and understandable results for the purpose of improving science literacy throughout the world.

Fig. 1.3.1 IMAGE integrated mission concept uses the FAST spacecraft architecture with existing Explorer ground systems.

A crucial part of the investigation is the proper deconvolution of the acquired images and their adoption into well-developed magnetospheric models through which their full impact on the discipline can be realized. In the remainder of this section the approach to instrument selection and science closure are discussed in order to establish clearly the investigation methodology for IMAGE.

1.3.1 IMAGE Instrument Requirements. Table 1.3.1 lists the measurement requirements of the IMAGE Baseline Mission along with the imagers that meet those requirements. The imagers shown between the double horizontal lines represent the Minimum Science Mission.

ImagerMeasurementCritical Measurement Requirements
Neutral atom composition and energy-resolved images from: 10-300 eV FOV -90o x 90o.
Angular Res. - 8o X 8o.
Composition - distinguish H, He and O in ion outflow and interstellar neutrals.
Energy Resolution - 0.8.
Image Time - 5 min.
Sensitivity - Effective area > 1cm2.
Neutral atom composition and energy resolved images.
MENA - 1-30 keV
HENA - 10-200 keV
FOV - 90o x 90o (image ring current from apogee.)
Angular Res. - 8o X 8o.
Composition - seperate solar wind (H) and magnetospheric (0) sources.
Energy Resolution - 0.8 (MENA), 0.7 (HENA).
Image Time - resolve substore processes (5 min.)
Sensitivity - Effective area > 1cm2 (See Fig. 1.2.4).
EUV 30.4 nm imaging of plasmasphere He+ column densities FOV - image plasmasphere from apogee (90o x 90o).
Spatial Resolution - 0.1 Earth radii from apogee.
Image Time - resolve plasmaspheric processes (several min - hours).
FUV Far ultraviolet imaging of the geocorona and neutral H and the auroral oval FOV - image auroral oval from apogee (8o).
Spatial Res. - 50 km.
Spectral Resolution - Separate cold geocorona H from hot proton precipitation ([[Delta]][[lambda]]~2 nm near 121.6 nm); separate 130.4 nm and 135.6 nm electron aurora emissions.
Image Time - resolve auroral activity (2 - 5 min)
RPI Radio sounder to measure electron densities and locate magnetospheric boundaries Image Time - resolve changes in boundary locations (1 min ).
Spatial resolution - resolve density structures at the magnetopause and plasmapause (500 km).
Density range - determine electron density from magnetopause to inner plasmasphere (0.1-105 cm-3)
Table 1.3.1 Summary of IMAGE measurement requirements. The Minimum Science Mission is represented by the instruments and measurements shown between the double horizontal lines.

1.3.2 Science Closure and Analysis. The IMAGE data will be used to answer the key science questions posed in Section 1.2. In the following, we give examples of the analysis of time sequences and the correlation of the images from different instruments that will be performed.

a. Are the structures in the cusp indicative of pulsed reconnection? This question will be answered by combining images from two instruments as a function of time. When the satellite is at high altitude in a position for the RPI to view the cusp, it will provide pictures of the shape of the cusp with high time resolution. The FUV will show us the footprint in the ionosphere. Figure 1.3.2 shows a simulated proton aurora image from the FUV spectrometer for moderate magnetospheric activity (Kp=4). The wavelength range is 121.6-122.8 nm. Local noon is down, and the cusp is clearly visible as an isolated emission patch centered on noon. When the satellite is at low altitude and flying near the cusp, NAI will provide a different picture of the cusp, but one that can also be correlated with the FUV images to determine its dynamics, whether steady or pulsating.

Figure 1.3.2 A simulated proton aurora image from the FUV spectrometer for moderate magnetospheric activity (Kp=4).

b. What are the processes that determine the structure and dynamics of the plasmasphere and plasmapause? The EUV images will show the structure of the plasmasphere directly (Figure 1.3.3). A 3-D, time-dependent image of the plasmasphere will be obtained by applying the deconvolution techniques described in the next section for NAI to these EUV images and combining the result with deconvolved RPI images. The deconvolved NAI images will give the position of the ring current relative to the plasmapause.

In the RPI data the quantity measured is the time delay, which is related to the path integral of the index of refraction. Because the index of refraction is a nonlinear function of the density, the deconvolution must be done iteratively. For most of the magnetospheric echoes to be obtained by IMAGE, the sounding frequency will be well above the plasma frequency over most of the propagation path. The iteration therefore converges very rapidly, yielding accurate density profiles.

The simultaneous FUV images will link the plasmapause shape with auroral activity. By combining the information from all these images, we will not only determine for the first time the global structure of the plasmapause but also its connectivity to other regions.

Fig. 1.3.3 Simulated EUV image of plasmasphere from the IMAGE spacecraft at an altitude of 7 Earth radii over the north pole.Based on model of Rasmussen et al., 1993.

c. How and where is plasma injected into the inner magnetosphere or ring current? NAI images are line-of-sight integral measurements, which can be deconvolved to produce ion pitch-angle distributions at the equator. The first step in this process is to expand the sought-after function in terms of coefficients of separable expansion functions. The second step is to use a regularization procedure based upon Bayesian statistics and the principle of minimum cross entropy to determine the coefficients of the expansion by satisfying a variational equation. This procedure has been well-developed within our team and has been applied to many different sets of simulated NAI images.

An example of the results is shown in Fig. 1.3.4, in which the simulated ion fluxes were produced using the 3-D storm model of Fok et al. [1995] with noise proportional to the square root of the counts included The pixels in the NAI images are 8 x 8 degrees with a sensitivity representative of the proposed NAI instrument. The perspective is from the noon-midnight meridian plane near apogee at 7 RE at midnight at a latitude of 45 degrees. The fourth row shows the deconvolved equatorial ion distributions integrated over pitch angle. The white areas are the peak fluxes, which exceed the values shown on the color bar. The deconvolved results clearly show (1) an enhancement of the ion flux in the injection with time, (2) the inward motion of the peak flux with time, and (3) the spread of the flux toward the dayside with time. In the bottom row, equatorial distributions for three different pitch angles at time 3 are shown.

Figure 1.3.4 Model ion source, neutral atom fluxes, simulated NAI images, and deconvolved ion source for 1.7 keV H during a magnetic storm on May 2, 1986 from a satellite at 00 MLT, radial distances 6.4 RE, 61 degrees latitude (Time 1, 08 UT), 7.0 RE, 45 degrees (Time 2, 10 UT) and 6.4 RE, 29 degrees (Time 3, 12 UT). First Row. Model equatorial ion sources out to 6.5 RE. Second row. ENA fluxes in a 90 by 90 degrees FOV. Third row. Counts as recorded in the IMAGE NAI instrument. Fourth row. Deconvolved equatorial ion flux integrated over pitch angle, to be compared with the model input shown in the first row. Fifth row. Deconvolved ion distributions for three pitch angles at Time 3.